Heterodimerization with β2-Adrenergic Receptors Promotes Surface Expression and Functional Activity of α1D-Adrenergic Receptors

The α1D-adrenergic receptor (α1D-AR) is a G protein-coupled receptor (GPCR) that is poorly trafficked to the cell surface and largely nonfunctional when heterologously expressed by itself in a variety of cell types. We screened a library of approximately 30 other group I GPCRs in a quantitative luminometer assay for the ability to promote α1D-AR cell surface expression. Strikingly, these screens revealed only two receptors capable of inducing robust increases in the amount of α1D-AR at the cell surface: α1B-AR and β2-AR. Confocal imaging confirmed that coexpression with β2-AR resulted in translocation of α1D-AR from intracellular sites to the plasma membrane. Additionally, coimmunoprecipitation studies demonstrated that α1D-AR and β2-AR specifically interact to form heterodimers when coexpressed in HEK-293 cells. Ligand binding studies revealed an increase in total α1D-AR binding sites upon coexpression with β2-AR, but no apparent effect on the pharmacological properties of the receptors. In functional studies, coexpression with β2-AR significantly enhanced the coupling of α1D-AR to norepinephrine-stimulated Ca2+ mobilization. Heterodimerization of β2-AR with α1D-AR also conferred the ability of α1D-AR to cointernalize upon β2-AR agonist stimulation, revealing a novel mechanism by which these different adrenergic receptor subtypes may regulate each other's activity. These findings demonstrate that the selective association of α1D-AR with other receptors is crucial for receptor surface expression and function and also shed light on a novel mechanism of cross talk between α1- and β2-ARs that is mediated through heterodimerization and cross-internalization.